I'm trying to design a PID controller but I have a problem in the following case:
The process variable can only be between 0 and 1. The setpoint can be close to one, for instance 0.97. Hence the lowest possible error is -0.03 (0.97 - 1), and the maximal error 0.97 (0.97 - 0). As a result, the controller output usually decreases much more slowly than it increases.
I would like it to be able to decrease faster, with the same set point. Is there a correct way to do that? Of course I could multiply the error by some constant only when it's negative, but I'm not sure it's right to do that (I don't have a lot of experience in control theory).